In recent years, mobile communications has developed significantly, and in Europe, mobile communications terminal apparatuses (hereinafter referred to simply as mobile terminals) using several frequency bands and several communications schemes are proposed. For instance, there is UMTS (Universal Mobile Telecommunication System) which combines GSM (Global System for Mobile Communications) using a frequency band of 900 MHz band, DCS (Digital Cellular System) using a frequency band of 1.8 GHz and WCDMA (Wideband Code Division Multiple Access) using a frequency band of 2 GHz.
Because UMTS needs to operate as a mobile terminal of a multi-system with differing frequency bands as well as communications schemes, the circuitry thereof becomes complex, and the increase in the number of components causes the apparatus to become larger and an increase in cost. For this reason, there is a need to reduce the number of components by integrating the circuitry, and to proactively share the components. In particular, since facilitating the sharing of an antenna, which is large in size, greatly contributes to the miniaturization of a terminal, the development of a small-sized switch apparatus is now an important issue in order to switch an antenna between systems.
As a conventional example, there is a switch apparatus for sharing an antenna using a diplexer and a high-frequency switch in a mobile terminal in which GSM and DCS are integrated. Switch apparatuses of this sort include the switch apparatus disclosed in Unexamined Japanese Patent Application No. 2000-183780, for instance.
The configuration of such a switch apparatus of the conventional example is shown in FIG. 4. An antenna, which resonates with the frequency bands of both GSM and DCS, is connected to an antenna terminal Tant. A GSM reception circuit is connected to a GSM reception signal terminal Tgsmrx. A GSM transmission circuit is connected to a GSM transmission signal terminal Tgsmtx. A DCS reception circuit is connected to a DCS reception signal terminal Tdcsrx. A DCS transmission circuit is connected to a DCS transmission signal terminal Tdcstx. A diplexer 1 has the combined characteristics of a high-pass filter and a low-pass filter, and mutually separates signals of GSM and DCS with an isolation of approximately 20 dB. A filter 2 attenuates the higher harmonics of a GSM transmission signal. A high-frequency switch 4 switches between transmission and reception of GSM. A filter 3 attenuates the higher harmonics of a DCS transmission signal. A high-frequency switch 5 switches between transmission and reception of DCS.
Here, PIN diodes are used for the high-frequency switches 4 and 5. The PIN diodes come to be in an on state when forward-biased and the impedance drops to approximately several Ω, and comes to be in an off state of high impedance with an inter-terminal capacitance on the order of several hundred fF. In general, in handling large power with a PIN diode, there is a higher probability of causing distortion in signals during transmission when it is in the off state.
In transmitting with GSM, a voltage of 3V is applied to control terminals Tctl2, and Tctl3, and 0V is applied to Tctl1 and Tctl4. As a result, PIN diodes D1 and D2 come to be in the on state, and D3 and D4 come to be in the off state. Thus, it is turned on between the GSM transmission signal terminal Tgsmtx and the filter 2, and the GSM transmission signal is transmitted to the antenna terminal Tant via the high-frequency switch 4, the filter 2 and the diplexer 1. Although part of the GSM transmission signal reaches the high-frequency switch 5, because it is attenuated by 20 dB or more from 33 dB of the GSM standards, only approximately 13 dBm is applied to the PIN diodes D3 and D4 both in the off state, and no distortion occurs even with a reverse-bias potential of a low voltage. In transmitting with DCS, avoltage of 3V is applied to the control terminals Tctl1 and Tctl4, and 0V is applied to the control terminals Tctl2 and Tctl3, but otherwise an explanation of the operation would be the same as GSM transmission.
Thus, the switch apparatus of the conventional example prevents the occurrence of distortion at the PIN diodes in the off state by mutually separating the transmission signals of GSM and DCS with the diplexer 1. It may be said that this is a configuration realizable with a small-sized diplexer by virtue of the fact that the frequencies are widely separated, GSM being in the 900 MHz band, and DCS in the 1.8 GHz band.
However, in order to make it possible to switch between WCDMA in addition to GSM and DCS, because WCDMA is in the 2 GHz band and DCS is in the 1.8 GHz band, and the frequencies are close to each other, there arises a problem in that these two signals cannot be separated with a small-sized diplexer.
Although a method of suppressing the occurrence of distortion by applying a reverse-bias potential of a high voltage to a PIN diode is also conceivable, it would become necessary to provide a voltage transformer apparatus such as a DC-DC converter, and there arises a problem in that the number of components and the cost increase. Therefore, this method is not suitable for a small-sized mobile terminal.
In addition, high-frequency switches such as the high-frequency switches 4 and 5 which use PIN diodes have a problem in their structure such that they are difficult to expand for use in switching between signals of 3 or more systems.
The present invention is provided in order to solve the this problem, and its object is to provide a small-sized switch apparatus capable of handling high-powered high-frequency signals with a relatively low control voltage, which switches between signals paths of at least a first, second and third frequency bands, and a mobile terminal apparatus using same.